Pyrene-labeled poly(N,N-dimethylacrylamide)s were prepared by radical copolymerization of N,N-dimethylacrylamide with N-(1-pyrenylmethyl)acrylamide. The progress of the copolymerization reaction was followed by 1H NMR to ensure that the pyrene-labeled monomer was homogeneously incorporated into the polymer backbone. Since pyrene is hydrophobic and poly(N,N-dimethylacrylamide) is water-soluble, a set of water-soluble associative polymers was generated. The effect of solvent quality toward these associative polymers was investigated. The level of association and the kinetics of encounter between pendants were determined in N,N-dimethylformamide (DMF), acetone, water, and mixtures of acetone and water. Analysis of the fluorescence decays with a blob model yields quantitative results which agree with the qualitative information retrieved by other techniques (static light scattering, intrinsic viscosity, UV−vis absorption). The level of association between hydrophobic pendants was found to be small in acetone (a good solvent for pyrene). It increases when water is added to the solution, since pyrene is insoluble in water. The level of association is smaller in DMF than in acetone, because DMF is a better solvent than acetone for the polymer, and swelling of the polymer coil results in a decrease of the interactions existing between the pyrene groups.

Two polyethylene glycols (PEG, M = 35 000) end-capped with short fluorocarbon tails were synthesized and characterized. In aqueous solution, the fluorocarbon portions associate strongly to form micelle-like structures which are bridged by PEG chains to form a three-dimensional network. As a result, these polymers in solution exhibit unusual rheological properties as a function of fluorocarbon length, polymer concentration, and shear rate (frequency). Their zero-shear viscosity increases with concentration, a common behavior of associating polymers. The viscosity is dramatically enhanced by replacing the end hydrophobe C6F13 with C8F17, a consequence of the stronger association interaction of C8F17 in aqueous solution. The polymer with the longer end group exhibits strong shear thinning once a critical shear rate is reached, whereas for the C6F13 end-capped polymer, we cannot with our equipment reach the shear-thinning regime. Our data indicate that between 2 and 6 wt %, and perhaps over a wider range of concentrations, both systems can be characterized in terms of identical values of the plateau modulus G N°, implying a similar concentration of chains bridging micelles in each system. The G N° values increase strongly with polymer concentration, consistent with a larger fraction of bridging chains and a smaller fraction of looping chains at elevated concentration. The viscosity difference between the two polymers can be explained in terms of a slower exit rate of the longer fluorocarbon from its micelle.

The "blob" model, developed to analyze the fluorescence decays of polymers randomly labeled with pyrene, has been applied to a series of pyrene-labeled poly(glutamic acid)s (PyPGA) in DMF and carbonated buffer solutions at pH 9. Poly(glutamic acid) (PGA) exists in the ionized form in the buffer solutions as poly(sodium glutamate) (PGNa). PGA adopts an alpha-helical conformation in DMF, whereas in aqueous solution PGNa is a random coil. Fluorescence, UV-vis absorption, and circular dichroism measurements indicate that in our studies pyrene pendants attached themselves along PGA in a clustered manner. Simulations were carried out to establish that the geometry of the PGA alpha-helix induces the high level of pyrene clustering. Since the level of pyrene clustering decreased with lower pyrene content, information about naked PGA was retrieved by extrapolating the trends obtained by fluorescence to zero pyrene content. Analysis of the fluorescence decays demonstrated that during its lifetime an excited pyrene probes a 32 amino acid section of the PGA alpha-helix. This result was supported by molecular mechanics optimizations. This study establishes that the blob model, originally used to monitor the encounters between pyrenes attached randomly onto a polymer adopting a random coil conformation, can also be applied to study the dynamics of the side chains of structured proteins. Since the blob model helps in monitoring the encounters between amino acids in the initial state (i.e., random coil) and in the final state (i.e., structured protein) of the folding pathway of a protein, it could be applicable to the study of protein folding.

The fluorescence decays of the pyrene monomer of a series of pyrene-labeled poly(N,N-dimethylacrylamide)s (PyPDMAAm) were acquired in N,N-dimethylformamide (DMF) and acetone with different concentrations of nitromethane and at very low polymer concentration. DMF and acetone are good and mediocre solvents for PyPDMAAm, respectively. Nitromethane is a potent quencher of pyrene. Nitromethane addition to the polymer solution shortens the pyrene lifetime. The fluorescence decays of the quenched and unquenched pyrene-labeled polymers were analyzed with a blob model. Shortening the lifetime of pyrene reduces the volume probed by the dye while it remains excited. For each quencher concentration, the blob volume and size were determined, and scaling laws were shown to hold. The exponents retrieved from the scaling relationships agreed with those reported in polymer science textbooks.